The 22-year-old was captured in November in the Russian border region of Kursk, where Ukraine launched an offensive in August. Since then, he has been held in Russian custody and now risks decades in Russian prison.
According to the Russian investigators, he crossed the armed border into Russia illegally and committed "criminal acts against the peaceful population".
The Briton has been shown with a shaved head and bound hands in several videos since he was captured.
The British Defence Minister David Lammy promised in November "all the support we can give" to the 22-year-old.
